The present invention relates to magnetic recordingplayback apparatus. More particularly, it relates to phase sensor means and method for a playback circuit calibrating means.
In the art of high performance magnetic tape recording, in the playback of recorded signals, there is a characteristic 6db per octave rise in the response of the system, up to a peak at a predetermined frequency, then a drop off of the response. Since that calibration is undesirable in the playback, means have been provided for compensating for that rise and fall characteristic by circuits known as equalizers. These equalizers, together with the preamplifier associated with the playback transducers, or heads, produce a response curve which is relatively flat from a predetermined minimum frequency to a predetermined maximum frequency known as "bandedge". At the bandedge frequency, there is a sharp decrease or rolloff in the response. The equalizers, in association with the playback heads, and, to some extent, the preamplifier, introduce a phase shift distortion which is particularly undesirable in digital recording systems. The phase shift distortion is eliminated or minimized by properly tuning the equalizer with the playback head and its preamplifier.
A square wave signal of a frequency equal to one third of the bandedge frequency has been recorded on the tape, then, on playback the resultant waveshape will be a wave with two peaks representative of the resultant of the first and third harmonic of the recorded frequency. That waveshape is viewed on an oscilloscope and the equalizer tuned to make the two peaks equal in amplitude.
It has now become desirable to include the tuning of the equalizer as a part of an automatic calibration system for such tape systems, wherein the calibration is accomplished by means of a microprocessor. When the equalizer is not properly tuned, either the first peak or the second peak will be higher in amplitude than the other. A difficulty that has been encountered is the absence of a suitable means for determining or measuring the relative amplitude of the two harmonic peaks as a measure of the tuning of the equalizer. Conventional sample and hold circuits were considered to be not suitable because the inescapable jitter in the tape causes the signal to be unstable to the point of uselessness.